1. Field of the Invention
The present invention relates to optical communication systems, and is more particularly related to an Erbium Doped Fiber Amplifier (EDFA) for minimizing four-wave mixing cross-talk.
2. Discussion of the Background
The explosion of communication services, ranging from video teleconferencing to electronic commerce, has spawn a new era of personal and business interactions. As evident in the enormous growth of Internet traffic, consumers have begun to embrace information technology, viewing it, in some cases, as much a necessity as the telephone. However, this new mindset poses many challenges to the telecommunication industry to develop technology that will greatly expand the bandwidth limitations of existing communication systems. Optical communications holds great promise to meet the continual demands for greater and greater bandwidth.
Wavelength Division Multiplexing (WDM) technology, in particular Dense WDM (DWDM), permits the concurrent transmission of multiple channels over a common optical fiber. The advent of Erbium Doped Fiber Amplifiers (EDFA) has accelerated the development of WDM systems by providing a cost-effective optical amplifier that is transparent to data rate and format. EDFAs amplify all the wavelengths simultaneously, enabling the composite optical signals to travel large distances (e.g., 600 km) without regeneration. Further, many advances in EDFA components and performance are underway to increase the bandwidth of EDFAs even more.
Not surprisingly, Internet services over WDM systems have garnered tremendous attention in the telecommunications industry. Notably, the industry has focused on the design of ultra-long systems (e.g., 1500-3000 km) that are capable of transporting high number of channels without optical regeneration.
L-band EDFAs are attracting a great deal of attention to increase the capacity of WDM optical communication systems. The physical mechanism of these amplifiers is substantially different from the conventional band (i.e., C-band) EDFA. In particular, they operate at low average inversion, and thus, require use of longer active fibers. However, four-wave mixing (FWM) efficiency within the active fiber is increased, leading to numerous serious system penalties. The effect is particularly detrimental in ultra-long-haul WDM systems in which there are many cascaded amplifiers.
The generation of FWM cross-talk (XT) within the EDFA differs from that of a transmission fiber. This is mainly due to the fact that the nonlinear effective length in the EDFA is much longer than the active fiber length. As a result, this gives rise to complicated mechanisms concerning the dependence of the FWM efficiency from channel separation, channel number, fiber length, signal power, and chromatic dispersion.
EDFAs operating in C-band are much less affected by this problem because of the shorter active fiber length.
Based on the foregoing, there is a clear need for minimizing FWM XT while maintaining a low noise figure by providing an optimized L-band EDFA with all-optical-gain-control (AOGC).
There is also a need to perform all-optical-gain-control with reduced transient power excursion in surviving channels induced by adding/dropping of channels.
There is also a need to reduce the length of the active fibers.
There is a further need to minimize the non-linear effects of the active fibers.
Based on the need to improve the performance of an optical communication system, an approach for reducing FWM cross-talk in an L-band EDFA is highly desirable.
According to one aspect of the invention, a method for minimizing four-wave mixing cross-talk in a WDM (wavelength division multiplexing) optical communication system is disclosed. The method includes injecting a co-propagant pump light into a first active fiber to induce high inversion in a portion of the first active fiber. The method also includes filtering WDM signals from the first active fiber to invert the gain tilt of the WDM signals. The method further encompasses injecting a counter-propagant light into a second active fiber to induce high inversion in a portion of the second active fiber. Under this approach, the FWM XT is reduced, along with the noise figure.
According to another aspect of the invention, an Erbium doped fiber amplifier (EDFA) apparatus operating in an L-band of a WDM communication system comprises a pre-amplifier that is configured to receive an input optical signal and to output an amplified input optical signal. The pre-amplifier includes the following components: a first active fiber that carries WDM signals; a co-propagating pump laser that is configured to induce high inversion in a portion of the first active fiber; a filter that is coupled to the first active fiber and configured to invert gain tilt of the WDM signals; a second active fiber that is coupled to the filter; and a counter-propagating pump laser that is configured to induce high inversion in a portion of the second active fiber. An optical unit may be coupled to the pre-amplifier and is configured to receive the amplified input signal, in which the optical unit introduces a prescribed loss to the WDM system. A booster amplifier is coupled to the optical unit and is configured to amplify a signal that is output from the optical unit. The above arrangement advantageously minimizes the non-linear effects of the active fiber.
According to yet another aspect of the invention, optical amplifier device for minimizing four-wave mixing cross-talk comprises a first active fiber configured to carry wave division multiplexing (WDM) signals corresponding to an operating band. A co-propagating pump laser is configured to induce high inversion in a portion of the first active fiber. A filter is coupled to the first active fiber and is configured to invert the gain tilt of the WDM signals. A second active fiber is coupled to the filter. A counter-propagating pump laser is configured to induce high inversion in a portion of the second active fiber. The above arrangement advantageously improves noise figure performance.
According to yet another aspect of the invention, optical amplifier apparatus operating in an L-band of a WDM communication system comprises a pre-amplifying means for receiving an input optical signal and for outputting an amplified input optical signal. The pre-amplifying means comprises a first active fiber that carries wave division multiplexing (WDM) signals, a co-propagating pumping means for inducing high inversion in a portion of the first active fiber, a filtering means coupled to the first active fiber for inverting tilt of the WDM signals, a second active fiber coupled to the filtering means, and a counter-propagating pumping means for inducing high inversion in a portion of the second active fiber. The apparatus also includes means coupled to the pre-amplifying means for receiving the amplified input signal and for introducing a prescribed loss to the WDM system. An amplifying means amplifies a signal that is output from the means for introducing the prescribed loss. The above arrangement reduces other non-linear effects within the active fiber, such as cross-phase-modulation.